FIG. 1 is a block diagram showing a configuration of a conventional direct-current power supply device described in Japanese Patent Application Publication No. 2012-23294. The direct-current power supply device shown in FIG. 1 is applied to an LED lighting apparatus. The LED lighting apparatus includes a power supply device 1 and a parallel circuit which includes a smoothing capacitor Co and an LED group load device RL and which is connected to the power supply device 1. An alternating-current power from an alternating-current power supply AC is converted to a direct-current power to be outputted to the LED group load device RL.
The power supply device 1 includes the alternating-current power supply AC, an EMI filter FL1, a full-wave rectifier circuit RC1, a smoothing capacitor Ci, diodes D1 to D3, a reactor L1, a control IC 100, and a capacitor C1. The EMI filter FL1 is connected to both terminals of the alternating-current power supply AC and an input terminal of the full-wave rectifier circuit RC1 configured to rectify an alternating-current voltage of the alternating-current power supply AC is connected to an output terminal of the EMI filter FL1.
A series circuit of a current detecting resistor Rs and a switching element Q1 including the regeneration diode D1 and a MOSFET is connected to an output terminal of the full-wave rectifier circuit RC1. A series circuit including: a main winding Np of the reactor L1; and the parallel circuit of the smoothing capacitor Co and the LED group load device RL is connected both terminals of the regeneration diode D1.
The control IC 100 turns the switching element Q1 on and off by using an on-off drive signal. The control IC 100 directly detects a load current flowing in the LED group load device RL by using a high-side current detecting unit 20 and inputs the detected load current into a current control error amplifier as a feedback signal FB via a level shift circuit, photocoupler, or the like which is not illustrated, in order that the detected load current can be equal to a predetermined average current value.
An output signal of the current control error amplifier 13 has a response characteristic slower than a half cycle of a sine wave of the alternating-current power supply AC. The PWM comparator 14 turns the switching element Q1 on and off while comparing the output from the current control error amplifier 13 and a triangle wave signal from a triangle wave generator 12 to each other to maintain a constant on-signal width of the switching element Q1.
Accordingly, a peak value of a switching current flowing in the switching element Q1 increases and decreases in a manner similar to a full-wave rectified voltage waveform of the sine wave of the alternating-current power supply AC. The direct-current power supply device thus also has a power-factor improving function.
Moreover, the control IC 100 detects, by using a comparator 11, voltage drop at an end of a regeneration current flowing in the main winding Np of the reactor L1, via the diode D3 based on a voltage signal of an auxiliary winding Nd of the reactor L1. Then, the control IC 100 resets a capacitor voltage of the triangle wave generator 12 and generates turn-on of the switching element Q1. The control IC 100 thereby performs quasi-resonant operation of the switching element Q1.
The conventional circuit shown in FIG. 1 is a low-side step-down chopper circuit in which the control IC 100 is driven with GND (ground) reference. Since the control signal can be inputted with GND reference, on-off operations and light adjusting operations by external signals can be facilitated.
However, since the LED group load device RL is connected to the high side where the voltage is high, the high-side current detecting unit 20 is provided as a circuit for sending the constant current control signal to the control IC 100 which is a GND reference circuit and the circuit configuration for sending the constant current control signal is complicated.
Meanwhile, when the control IC 100 is on the high side (=floating operation), the circuit configuration for the constant current control signal can be simple. However, since the destination of the external signals for the on-off and light adjusting operations is the control IC 100 on the high side, the circuit configuration of the control IC 100 is complicated.
As described above, regardless of being a high-side or low-side chopper circuit, the circuit of the conventional technique has a problem that the circuit configuration for the external control signal or the LED current detection signal is complicated and the cost of the device is high.